Hello Laurag,
Sorry for the trouble you had in viewing the comments in the PDF file, Open the file in Adobe Reader and then try opening the Comments list from Tools>Comment and the Comments list will be displayed in the right pane.
From the options menu at the upper-right corner of the comments list, expand the comments and check are you able to see the comments?
If the comments lists are not appearing in the list, then probably the PDF file sent to you is flattened by someone.
Try adding any comments in the PDF and check if you can see the comments in the sidebar. Navigate to the Adobe Reader's Preferences from Edit>Preferences>Commenting, under Pop-Up open behavior select "Automatically open comment pop-ups when comment list is open", Click OK and see if this makes any difference.
Make sure that you have the latest version of Adobe Reader installed, check for any pending updates of Adobe Reader from help>check for updates, reboot the machine after updating Adobe Reader.
Have you tried with other PDF files or is it specific to one PDF file? What is the dot version of Adobe Reader installed? To identify refer to Identify the product and its version for Acrobat and Reader DC
What is the version of operating system installed?
Let us know how it goes and share your findings.
Thanks,
Anand Sri.